In the graphic arts, it is desirable to produce a three or more color proof to assist in correcting a set of color separation films prior to using them to produce printing plates. The proof should reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplicate of the desired halftone image. Visual examination of a color proof should show the color rendition to be expected from press printing using the color separations and any defects on the separations which might need to be altered before making the printing plates.
Color proofing sheets for multicolored printing can be made by using a printing press or proof press. This requires that all of the actual printing steps be performed. However, this conventional method of color proofing is costly and time consuming.
Photoimaging processes can also be used to produce color proofs. There are two general types of photoimaging methods; namely the overlay type and the single sheet type.
In the overlay type color proofing method, an independent transparent plastic support is used for producing an image of each color separation film by applying a photosensitive solution of the corresponding color. A plurality of such supports carrying images of the corresponding colors are then superimposed upon each other over a white sheet to produce a color proofing composite. The primary advantage of the overlay method is that proofs can be made quickly and can serve as a progressive proof by combining any two or three colors in register. However, this type of color proofing method has the disadvantage that the superimposed plastic supports tend to darken the color proofing sheet. As a result, the impression of the color proofing composite thus prepared is vastly different from that of copies actually obtained with conventional printing presses and with proof presses. Examples of such overlay approaches are contained in U.S. Pat. Nos. 3,136,637; 3,211,553; and 3,326,682.
In the single sheet type of color proofing method, a color proofing sheet is prepared by successively producing images of different colors from different color separation films onto a single receiver sheet. This is done by utilizing a single opaque support and by applying toners, transferring images, or processing solutions or coatings of photosensitive materials of corresponding colors on the opaque support in succession. An advantage of the single sheet type of color proof is that the color saturation is not influenced by superimposed plastic supports. This method more closely resembles the actual printing process and eliminates the color distortion inherent in the overlay system.
The present invention relates to improved negative-working and positive-working proofing sheets of the single sheet, solid layer transfer class. Color proofing films of this type are composed of a sheet substrate, a colored photosensitive layer on the sheet, and an adhesive layer disposed on the photosensitive layer.
An example of a negative-working, single sheet approach is described in U.S. Pat. No. 3,671,236. The presensitized element comprises a substrate having a release surface, a pigmented photosensitive layer in contact with the release surface, a pressure sensitive adhesive layer, and a barrier layer between the photosensitive layer and the adhesive layer. The primary function of the barrier layer is to prevent pigment staining on the adhesive during development. The element is used by laminating it to a receiver via the adhesive layer. The substrate is removed, the photosensitive layer is exposed, and the material is processed in a solvent medium. These steps are generally repeated three times to produce a multicolored proof.
An example of a positive-working, single sheet approach is given in U.S. Pat. No. 4,260,673. The presensitized element comprises a substrate having a release surface, a pigmented photosensitive layer in contact with the release surface, a heat sensitive adhesive layer, and a binder layer between the pigmented layer and the adhesive layer. The binder layer comprises a photosensitive compound and prevents pigment staining of the adhesive. The element is processed like the negative-working element in U.S. Pat. No. 3,671,236.
Examples of negative-working and positive-working, single sheet approaches without preventative layers between the photosensitive and adhesive layers are described in U.S. Pat. Nos. 4,650,738 and 4,659,642, respectively. The presensitized elements comprise a sheet having a release surface, a colored photosensitive layer in contact with the release surface, and an adhesive layer directly adhered to said colored layer. The adhesive layer comprises a vinyl acetate polymer.
This invention provides an improved photosensitive element which provides an adhesive layer which is partially removable by the liquid developer. In this manner the uppermost part of the adhesive layer is removed so that the layer retains its adhesive property but any residual photosensitive layer which causes background staining is removed.